Introduction

Often we find following assemble code

.align <n>

Where n is an integer which could be e.g. 3, 5

In this document, we will do some experiment for understanding this.

Test code

Create a file a.S with following content

mov x0,0xf5
.align 3
mov x0,0xf4

Compile

Now using an arm cross compiler to compile above code

aarch64-linux-gnu-gcc -c a.S

This will generate an object file a.o

pandy@ubuntu:~/temp/asm_test$ file a.o
a.o: ELF 64-bit LSB relocatable, ARM aarch64, version 1 (SYSV), not stripped

Disassemble the code

Let’s check what is the generated code:

pandy@ubuntu:~/temp/asm_test$ aarch64-linux-gnu-objdump -d a.o

a.o:     file format elf64-littleaarch64


Disassembly of section .text:

0000000000000000 <.text>:
   0:   d2801ea0        mov     x0, #0xf5                       // #245
   4:   d503201f        nop
   8:   d2801e80        mov     x0, #0xf4                       // #244

It is found that after .align 3, the code is put to the alignment of 8, which happens to be 2^3.

More testing

Let’s do some change:

mov x0,0xf5
.align 5
mov x0,0xf4

pandy@ubuntu:~/temp/asm_test$ aarch64-linux-gnu-objdump -d a.o

a.o:     file format elf64-littleaarch64


Disassembly of section .text:

0000000000000000 <.text>:
   0:   d2801ea0        mov     x0, #0xf5                       // #245
   4:   d503201f        nop
   8:   d503201f        nop
   c:   d503201f        nop
  10:   d503201f        nop
  14:   d503201f        nop
  18:   d503201f        nop
  1c:   d503201f        nop
  20:   d2801e80        mov     x0, #0xf4                       // #244

0x20 happens to be 2^5.